This invention pertains to an improvement of the oscillometric method for noninvasive blood-pressure measurement, and more particularly to a unique application for microprocessor-controlled blood-pressure monitoring.
Examples of noninvasive blood-pressure measurement methods in the closest prior art are disclosed in U.S. Pat. Nos. 4,461,266 to Hood, et al. and 4,638,810 to Ramsey.
In the typical practicing of an oscillometric, noninvasive blood-pressure measurement method with a person, a counterpressure-producing cuff is wrapped around the person's upper arm, with the cuff then inflated to a counterpressure above systolic pressure to occlude a artery (blood vessel) in the arm. Thereafter cuff counterpressure is progressively reduced in a stepped fashion from this suprasystolic pressure to an ending counterpressure where the cuff is substantially deflated. During progressive reduction of cuff counterpressure, the artery opens progressively from an occluded state to an unoccluded state.
During the change from the occluded state to the unoccluded state, the artery begins to pulsate against the cuff, and the waveforms of these pulsations are monitorable to produce graphic illustrations of blood-pressure parameters. As is well-known to those skilled in the art in handling blood-pressure measurements, the pulsations just referred to increase in amplitude toward a maximum as cuff counterpressure decreases below systolic pressure, and then decrease in amplitude. By categorizing these pulsations relative to their occurrences in time and to their respective amplitudes, desired blood-pressure parameters are determined.
Explaining the significant monitored heart activity with a little more particularity, during each heart contraction, a force is exerted upon the blood in the vascular system. During the time that this force is active, the blood is accelerated, or given momentum. The integral of this force with respect to time (when the force is active) is called the "impulse" of the force, with "impulse" bearing the same units as momentum. Accordingly, if the instantaneous pressure in the cuff is monitored during a measurement procedure, and integrated over the time during which measurements are being made, it is possible to develop a data quantity that is directly proportional to impulse--that characteristic of blood flow from which, it turns out, the most accurate blood-pressure data can be derived.
A critical determination is that of mean arterial pressure (MAP). It is from this determination that systolic and diastolic pressures are calculated. Typically, MAP has been defined according to the prior art as the pressure in the cuff where blood-pressure pulsations have the largest amplitudes. Amplitude data, however, does not relate well to the characteristic described above as impulse, and, because pulsation impulse data is, for the sake of ultimate accuracy, the most desirable data, it does not reliably produce the most accurate ultimate information.
The method of the present invention significantly addresses this issue.
Another consideration is that conventional blood-pressure measuring methods typically reduce counterpressure, progressively, in steps at a relatively slow rate. This results in a relatively long time period for an entire measurement cycle, and often as a consequence, patient discomfort.
Finally, in all methods of acquiring usable blood-pressure data, it is important to detect, and reject, as faithfully as possible, pressure "artifacts" which are not induced by blood-pressure pulsations. Artifacts occur, for example, where a subject moves, changes muscle tension, etc.
An important object of the present invention, accordingly, is to categorize blood-vessel pulsations in a far more accurate manner by a value that more closely approximates blood-vessel pulsation impulse.
Another object of the invention is to provide for artifact rejection in a unique way which ensures that accepted pressure waveforms truly are blood-pressure induced.
A further object is to decrease the number of pressure waveforms that are monitored at each cuff counterpressure level, thereby to decrease the overall time period of a measuring cycle, thus to minimize subject discomfort. The method of the present invention, which might be thought of as an "impulse-based method", offers a significant improvement over the closest prior art because, inter alia, it defines the blood-pressure pulsation which corresponds to MAP as that pulsation which produces a waveform having the greatest area, as distinguished from that having the greatest amplitude--area being a direct indication of impulse. Such waveform area data is an indicator of MAP which for many reasons is more accurate than waveform amplitude data.
Another extremely important consideration is that where waveform area (impulse) data forms the foundation for the determination of MAP, systolic pressure and diastolic pressure, signal-to-noise problems are greatly reduced.
For all of the important reasons given above, the desired blood-pressure parameters of a subject, determined in accordance with the present invention, have improved accuracy over the same parameters determined in accordance with the closest prior art.
To deal with the issue of false "artifact" data, the invention employs two different artifact-rejection techniques, during two different phases of a blood-pressure measuring cycle, to assure that developed waveform area data accurately and reliably represents blood-pressure-induced changes in the occluding cuff.
The first artifact-rejection technique verifies that monitored pressure signal data corresponding to pressure waveforms is blood-pressure induced.
A second artifact-rejection technique verifies that developed area data values are also blood-pressure induced. This second technique, after development of waveform area-data values for a predetermined number of cuff counterpressure levels at the beginning of a measuring cycle, predicts a next, expected-to-be-encountered area-data value for the next, lower cuff counterpressure level. Employing prediction for successive, next, lower cuff counterpressure levels, provides a simple and accurate method of artifact rejection that substantially decreases the number of pressure waveforms required to be monitored at a given cuff counterpressure level. Therefore, if a next, developed waveform area-data value for a measured waveform is within experimentally set upper and lower bounds of its corresponding predicted value, the measured value is accepted as being blood-pressure induced.
This important feature of area-data acquisition, coupled with on going next-to-be-expected value prediction, significantly enhances the likelihood that a false data pulse will be rejected as an artifact.
Using the prediction technique just described for subsequent cuff counterpressure levels, it will generally be necessary to monitor only one pressure waveform at a given cuff counterpressure level. As will be explained, if the first pressure waveform which is monitored does not have an area-data value that is within the upper and lower bounds of its corresponding predicted value, subsequent waveforms will be monitored until one is found which does meet the boundary conditions. This situation, of "looking" for successive, subsequent "boundary-meeting" waveforms, continues only for a predetermined ultimate time interval, after which, if no proper waveform is found, the method of the invention aborts the measurement cycle.
In addition, and further in accordance with special features of the invention, the second artifact-rejection technique adjusts previously encountered (and stored) waveform area-data values based on the difference between a measured waveform area-data value and a corresponding predicted waveform area-data value for a given cuff counterpressure level. This is referred to herein as a "smoothing" technique. By adjusting previously stored values, this second technique provides further ensurance of the accuracy of ultimately derived, desired blood-pressure parameters.
These and other objects and advantages which are attained by the invention will become more fully apparent as the description that now follows is red in conjunction with the accompanying drawings and computer program flow charts.